I have a 97 528i and it wouldn't start or turn over for a couple days then it would start for couple days now it won't.
I am sure it is a relay or electrical issue because of of the sudden nature.

My mechanic suggested it is the anti theft relay. Any comments?

If the EWS system is responsibe there'll be a fault code present. Other possibilities include a bad battery/starter connection, bad starter, bad immobilzer fuse, or ignition switch. A bit of detective work with a voltmeter should reduce the search area.